Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital data signals. There are various methods that can be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual ink droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. In another process, known as continuous inkjet, a continuous stream of droplets is charged and deflected in an image-wise manner onto the surface of the image-recording element, while unimaged droplets are caught and returned to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
The ink compositions known in the art of inkjet printing can be aqueous-based or solvent-based, and in a liquid, solid, or gel state at room temperature and pressure. Aqueous-based ink compositions are preferred because they are more environmentally friendly as compared to solvent-based inks, plus most printheads are designed for use with aqueous-based inks.
The ink composition can be colored with pigments, dyes, polymeric dyes, loaded-dye/latex particles, and other types of colorants, or combinations thereof. Pigment-based ink compositions are advantageously used because such inks render printed images having higher optical densities and better resistance to light and ozone as compared to printed images made from other types of colorants. The colorant in the ink composition can be yellow, magenta, cyan, black, gray, red, violet, blue, green, orange, brown, etc. Although a very numerous variety of ink compositions are known in the art of inkjet printing, several key challenges remain, especially the simultaneous achievement of various challenges in a single ink composition, in view of the tendency of many ink components to have both benefits and drawbacks. One challenge is to obtain the highest possible density on a variety of inkjet receivers.
Typically the receivers for ink compositions are categorized as a photo-quality or plain paper receivers. These two types of receivers are distinguished from one another in that the photo-quality receivers are more glossy and are manufactured with one or more coated layers above the underlying paper support. The one or more coated layers comprise at least an image-receiving layer. Typically, such photo-quality receivers can be further categorized as swellable receivers, having a non-porous polymer coating or microporous or “porous” receivers having a porous coating of particles held together with a minor amount of binder. Hybrid designs are also known. Typical swellable media are capable of very high gloss in excess of 60 gloss units at a viewing angle of 60 degrees. Typical microporous media can be designed to have gloss values approaching those of some polymer coated media and are quick to dry. The design of both plain paper and photo-quality media vary widely depending on materials and manufacturing processes.
Inkjet printing onto plain paper receivers presents an especially challenging problem, namely curl control. Photo-quality media, compared to plain paper media, can employ relatively thick backing layers to limit curl. Also, plain paper receivers are particularly susceptible to physical curling of the paper that can occur after aqueous based inks are applied and subsequently dried, because relatively large amounts of liquid ink, relative to the absorption capacity of plain paper media compared to photo-quality paper, are printed to a plain paper substrate, for example, when graphics images are printed.
Various ink additives have been disclosed in order to minimize curl of inks when applied to plain papers such as those disclosed in U.S. Pat. No. 5,356,464. However, the presence of curl reducing compounds in the ink formulation can have deleterious affects on the image density and image quality of the printed images. It is therefore a goal to have ink compositions that minimize image receiver curl while providing high image density. Furthermore, yet a third challenge for inkjet printing is the minimization of image quality artifacts related to bleed and coverage uniformity. The design of the ink components can have a significant impact on image quality artifacts, especially on certain types of receivers. Non-uniform ink coverage onto a particular ink receiver can result in both macroscopic and microscopic density fluctuations that are objectionable to the observer. Similarly, the design of ink components and the receiver can have a significant affect on ink bleed. Bleed occurs when two inks are printed adjacent to one another in close proximity and the inks mix while spreading and penetrating the receiver surface. This can have an adverse affect on color properties and sharpness of the final image. It is therefore a goal of ink compositions to minimize bleed and to have high coverage uniformity on a particular, and more preferably across a wide variety of, image receivers, while simultaneously reducing image receiver curl and providing high image density.
Ethoxylated glycerol compounds, and classes of compounds encompassing such compounds, have conventionally been used as humectants or the like in inkjet inks as disclosed in U.S. Pat. Nos. 5,169,437; 5,180,425; and U.S. Patent Publication No. 2003/0037699. Such compounds can be made from reacting glycerol or the like with alkylene oxide units. For example, U.S. Pat. No. 5,169,437 discloses the use of compounds having a certain structural formula, in which structural formula alkylene oxide chains are attached to a glycerol backbone, which compounds are used as a humectant to mitigate the problems arising with the crusting and clogging of orifice plates in thermal inkjet printers. The compounds were found to reduce the rate of evaporation of an aqueous ink composition. U.S. Pat. No. 5,180,425 to Matrick et al. discloses the use of compounds having a similar structural formula, in which structural formula alkylene oxide chains are attached to a backbone derived from a polyol having 3 or more hydroxyl groups such as glycerol, trimethylol propane, or the like, which compounds are used as a co-solvent to prevent film formation on resistor surfaces while maintaining stable pigment dispersions free from premature nozzle plugging. U.S. Patent Publication No. 2003/0037699 to Yatake discloses the use of compounds having a certain structural formula, in which structural formula both ethyleneoxy and propyleneoxy groups randomly or as blocks joined together to form alkylene oxide chains are attached to a glycerol backbone, which compounds are used as a humectant to provide an ink composition with excellent ejection stability, which ink composition further comprises glycerin and optionally further 1,2-alkylene glycol or di(tri)ethylene monobutyl ether.